The present invention relates to a control device for positioning an optical focus in of an optical disc apparatus that records, plays back, and erases a variety of information by irradiating optical beams, such as the laser beams, onto a recording media composed of magnetic film.
Conventionally, existing optical disc apparatuses easily cause the surface of an optical disc to vibrate during rotation, and as a result, recording tracks on the disc are displaced in the direction of the optical axis of the incident laser beams that irradiate the disc surface. Also, the disc is adversely affected by any deviation existing between the center position of the disc and the center of the motor shaft that drives the disc. Recording tracks of such a disc with this centering deviation are displaced in the direction of the disc radius (hereinafter called the radial direction). To prevent the recording tracks from being displaced from the incident laser beams a device is provided so that the position of the laser beam focus can be correctly adjusted within the optical head mechanism so that the incident laser beam spot correctly matches the recording tracks of a disc. Such a device is called a control device for positioning the optical focus in the following description.
Using any of the existing optical disc apparatus as such as one that only plays back information without containing any magnetic film recording media, or another that can record additional information, in order to precisely adjust the focus position of the incident laser beams (hereinafter called the focus controller) to compensation disc displacement in the direction of the optical axis of the incident laser beams, a device that varies the position of the objective lens of the incident laser beams by means of electromagnetism is well known. On the other hand, to precisely adjust the focus position of the incident laser beams (hereinafter called the tracking controller) to compensate for disc displacement in the radial direction, a variety of mechanism that can adjust the focus position of the incident laser beams via a rotary mirror that reflects the incident laser beams against any optimum direction have been introduced. A new proposal has been introduced quite recently, which provides a mechanism capable of jointly performing both the focus and tracking controls mentioned above, by varying the position of objective lens via the electromegnetic force. Basically, the mechanism comprises a coil that can be moved integrally with an objective lens and a stationary permanent magnet, thus causing the objective lens to be displaced by the current flowing through said coil.
If such an already known mechanism capable of jointly performing both the focus and tracking controls by varying the position of the objective lens via the electromagnetic force is actually applied to an optical disc apparatus, it will easily create problems as described below.
Since the proposed mechanism uses magnetism generated by a permanent magnet, a leakage magnetism or flux will be generated in portions peripheral to the disc. Nevertheless, since the disc uses the magnetic film for the recording media, if such a leakage magnetism adversely affects said megnetic film, the following problems will arise.
(1) When the laser beams are irradiated onto an optical magnetic disc they cause the disc temperature to rise, and simultaneously information is recorded on said disc via an external magnetism, however, if a leakage magnetism from said optical focus position control affects the disc, then the quality of the recorded information will be degraded significantly. PA1 (2) When playing back the recorded information via the magnetic-optical effect by irradiating laser beams onto the optical magnetic disc, any leakage flux from said optical focus position control may adversely affect the disc, thus causing the recorded information to be easily erased.
In light of these potential disadvantages, if an optical disc apparatus is used, it is quite necessary to completely prevent even the slightest leakage flux from the optical focus position control from seriously affecting the optical disc. In addition, there are still further problems to solve. For example, one might use a mechanism incorporating an objective lens-mirror cylinder which is supported by rubber material whose one end is secured to a stationary holder, while said objective lens-mirror cylinder can be driven by electromagnetic force existing between the coil secured to said objective lens-mirror cylinder and the magnetic circuit secured to said stationary cylinder. Since said rubber-elastic material supports the objective lens-mirror cylinder, it cannot fully resist the tilting force of the objective lens-mirror cylinder, and as a result, an accidental force may be generated when the drive force generated by said electromagnetic force cannot be applied to the gravity center of the objective lens-mirror cylinder, thus causing said cylinder to eventually generate a rotary movement. This will cause the optical axis of the incident laser beams to tilt against the center axis of the objective lens. The result is that either off-axis astigmation or coma aberration will adversely affect the disc tracks that contain information, causing beams to poorly focus on them, and as a result, quality of the recorded information will be degraded significantly.